2 symmetrical identical planets

Sometime ago, a guy asked about a scenario that if there were another earth planet opposite to our earth accross the sun, could we identify it.
Now, my question is if the two finally would coalesce after some time?

Is it possible that the two planets would finally colide because besides the sun, they also were influenced by other planets and these influences were not identical to the two. So the symmetry would be broken after sometime.

The odds of something large coming from outside the solar system are negligable. Oort Cloud objects are not massive enough to significantly disrupt anything.

If there were another Earth opposite our Earth it would spend its time jumping from trojan orbit to horseshoe orbit with Earth. They would not get very close to each other.

So why when the planets were formed, they were formed singularly. I means at the distance of Mercury, there is one Mercury, then one Venus, one Earth etc..
We know planets were formed from an accretion disc. If the 2 Earth could exist simultaneously, then there should have been many more smaller objects orbiting in the Earth's orbit now.